A stable supply of gases (e.g., oxygen gas or hydrogen gas) and a function of discharging water generated by a reaction between the gases are important characteristics of a separator for fuel cell. Particularly, rapid discharge of the generated water has much effect on characteristics of a fuel cell. One of important requirements for the smooth discharge of generated water is a hydrophilic property (wettability) of a surface of a rib of the separator for fuel cell.
A molded article containing a resin and a conductive filler is widely used for the separator for fuel cell, and the separator for fuel cell made from such a electroconductive resin readily repels water due to its high water repelling property. Water droplets deposited on a channel cause a reduction in circulation efficiency of the generated water to degrade the cell characteristics.
In order to improve the circulation of the generated water, various treatment for increasing hydrophilic property have heretofore been proposed. For example, a method of activating a surface by ozone treatment through UV irradiation has been known (see Reference 1). However, it is necessary to perform the UV irradiation for a considerable period of time in order to realize a sufficient hydrophilic property by the method, and the hydrophilic property achieved by the method changes largely with time to result in insufficient durability. Also, a method of forming a cured film by coating with a hydrophilic monomer has been known (Reference 2). However, flow of the cured film can cause a reduction in hydrophilic property, and an insulation property of the cured film can cause a reduction in electroconductivity. Further, a method of blending a hydrophilic resin such as a phenol resin and an epoxy resin as a binder has been known (Reference 3). However, hydrophilic property on a surface achieved by this method is insufficient. Furthermore, a blending of polymethacrylate as a hydrophilic agent has been proposed (Reference 4), but this method can cause a reduction in hot strength, an increase in creep, and a reduction in water resistance of the obtained separator for fuel cell.
Reference 1: JP 2000-243410 A
Reference 2: JP 2000-251903 A
Reference 3: JP 2000-311695 A
Reference 4: JP 2002-352813 A